In a telecommunication, quality of a connection between a base station and a user equipment is typically estimated by means of measuring on certain known signals. Downlink refers to transmission from the base station to the user equipment and uplink refers to transmission from the user equipment to the base station.
As an example of a known telecommunication network, a Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) network is considered. With the LTE network, the certain known signals are referred to as reference signals. Within LTE, various reference signals have been introduced. LTE uses Orthogonal Frequency Division Multiplexing (OFDM) to form a time-frequency structure. A time domain is divided into subframes, where each subframe, including two slots, is 1 ms. Each slot typically includes 7 OFDM symbols. A frequency domain is divided into subcarriers. In the time-frequency domain, a resource block is defined by 12 subcarriers in the frequency domain and one slot in the time domain. A resource element is defined by one subcarrier in the frequency domain and one OFDM symbol in the time domain.
Firstly, Cell-Specific Reference Signals (CS-RS) are described. The CS-RS are downlink reference signals transmitted in every downlink subframe in the time domain and in every resource block in the frequency domain. Thus, the CS-RS can cover an entire bandwidth of a cell. The CS-RS is used by the user equipments for channel estimation for coherent demodulation of any downlink physical channel, except for Physical Multicast Channel (PMCH), and for Physical Downlink Shared Channel (PDSCH) in the case of Transmission Modes (TM) 7, 8, and 9. For release 8 and 9 user equipments, it is also used by the user equipment to acquire Channel State Information (CSI).
Secondly, Channel State Information Reference Signals (CSI-RS) are described. The CSI-RS are intended for release 10 user equipments and release 10+user equipments in order to acquire CSI. Specifically, in release 10, the CSI-RS are used for transmission mode 9 (TM9). A reason for introducing CSI-RS is to improve channel estimation for coherent demodulation even with extreme channel conditions, including fast channel variations in both the time and frequency domains without introducing more overhead than necessary. The CSI-RS, which only targets CSI, has a flexible, and in general lower, time/frequency density than CS-RS. Thus, a lower overhead is introduced than if CS-RS would be extended to target CSI.
For TM9 user equipments, a signal is measured on CSI-RS. In TM9 interference is taken on the CRS resource as the sum of noise and other cells CRS interference or data interference.
The configuration of a resource for CSI-RS is performed with Radio Resource Control (RRC) signalling. A mapping of the resource for CSI-RS to a resource elements is defined in Table 6.10.5.2-1 for normal cyclic prefix and 6.10.5.2.2 for extended cyclic prefix in 3GPP TS 36.211 V11.3.0.
For TM9, two parameters need to be addressed in the RRC signalling, or RRC configuration, see 3GPP TS 36.331 V11.3.0, to configure CSI, CSI-RS and zeroTxPowerCSI-RS. It is possible to configure one CSI-RS per cell/sector. Basically, to measure the channel quality of CSI-RS of one cell/sector accurately, the neighboring cells/sectors' CSI-RSes have to be muted, i.e., configured with zeroTxPowerCSl-RS. Furthermore, in TM9, only one CSI report is supported.
Consider a known telecommunication network, which is described with reference to LTE, comprising a base station, or eNB, which operates a cell, such as a multi-sector cell. The cell includes two or more sectors. The sectors may be configured with the same or different resource for transmission, by the base station, of the CSI-RS. In the following, it is assumed that the cell includes a first sector and a second sector. User equipment is also located in the cell.
In a first scenario, a first and a second resource for CSI-RS are configured in the first and second sector, respectively. This means that different resources are configured in different sectors.
If the user equipment is located in an area that has good coverage from the first and second sectors, the user equipment can receive Physical Downlink Shared Channel (PDSCH) data from both sectors. The area with good coverage can be at an edge between the first and second sector, e.g. at any distance from the base station. However, a Channel Quality Indicator (CQI) report, from the user equipment, is based on CSI-RS will reflect transmission from only one of the sectors. A reason for this is that the user equipment can only support one CSI-RS configuration at any given time. This means that it is not possible for the user equipment to be configured with different resources for the first and second sectors in which it is located. Therefore, the CQI report is underestimated. As a consequence, the base station will assign the user equipment with a lower data rate than the user equipment actually is capable of. A disadvantage is thus that transmission in the multiple sectors according to TM9 is thus not fully exploited.
In a second scenario, a common resource for CSI-RS is configured in the first and second sectors. This means that the same resource is configured in different sectors.
Again, if the user equipment is located in an area that has good coverage from the first and second sectors, the user equipment can receive CSI-RS from both sectors, but the user equipment can only receive PDSCH data from one sector. In effect, the CQI report, generated by the user equipment, based on CSI-RS is overestimated. However, the base station is not aware of this inaccuracy of the CQI report. Therefore, the base station will assign, to the user equipment, a higher data rate than the user equipment actually is capable of. This leads to failed data transmission, which could cause overload the base station due to unnecessary retransmissions. In general, a disadvantage may be that an overall throughput may be degenerated.
In order to so reduce the above mentioned disadvantages, it has been proposed to change CSI-RS configuration in the user equipment when the selected sectors are changed. A problem in this regard is that an RRC reconfiguration is very costly, e.g. in terms of time and resources.
Furthermore, it has been proposed to configure the user equipment with a resource for one CSI-RS at a time, but time dynamically share the resource with different sectors in different time instances, such as time slots. This means that the base station transmits the same CSI-RS at the same resource from different sectors at different time instances. The base station can then obtain the channel quality measurement of different sectors by keeping track of the time instance and the measurement of CSI reports from the transmissions of different sectors at different time instances.
However, a problem is how to perform filtering of the CSI-RS from different sectors in the user equipment. Since the user equipment is not aware of the change of the transmitting sector, there is a risk that the measurement results from the different sectors are filtered together and the measurement results will be inaccurate.